#5689 Refactor enforcement of a single Score context

[Dan Eble]

Context::find_create_context () and Context::create_unique_context ()
no longer require special cases to avoid ascending to the Global context
or to create a Score themselves. Instead, the method that they each
call to create any new context, Context::create_hierarchy (), avoids
creating multiple Scores.

This is still hard-coded in C++ to happen for Score only, but it would
be straightforward to add a context attribute that could be used in ly
code like \accepts and \defaultchild.

https://codereview.appspot.com/551390046

[Anonymous]

passes make, make check and a full make doc

[Anonymous]

Patch on countdown for Jan 29th

[Anonymous]

Patch counted down - please push

[Dan Eble]

commit 5e40b2af5005e1fd1d0bee225b55ef544be80631
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Fri Jan 17 22:22:33 2020 +0100

    Issue 5689: Refactor enforcement of a single Score context

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